High-strength cold rolled steel sheet having excellent shear processability, and manufacturing method therefor

ABSTRACT

Provided is a high-strength cold rolled steel sheet having high shear processability, including, by wt %, C: 0.05% to 0.10%, Si: 0.01% to 0.5%, Mn: 1.2% to 2.0%, Al: 0.01% to 0.1%, Cr: 0.005% to 0.3%, B: 0.0003% to 0.0010%, Mo: 0.005% to 0.2%, P: 0.001% to 0.05%, S: 0.001% to 0.01%, N: 0.001% to 0.01%, Nb: 0.005% to 0.08%, Ti: 0.005% to 0.13%, V: 0.005% to 0.2%, and a balance of Fe and inevitable impurities. The high-strength cold rolled steel sheet satisfies Formulas 1 and 2 below, and the high-strength cold rolled steel sheet includes at least one of carbides, nitrides, and carbonitrides. Formula 1: 2.0≤[Mn]+2.5[Mo]+1.5[Cr]+300[B]≤2.5, Formula 2: 0.2≤([Nb]/93+[Ti]/48+[V]/51)/([C]/12+[N]/14)≤0.5 (in Formulas 1 and 2, each element symbol refers to a weight percent (wt %) of a corresponding element).

TECHNICAL FIELD

The present disclosure relates to a high-strength cold rolled steelsheet having high shear processability, and a method for manufacturingthe high-strength cold rolled steel sheet.

BACKGROUND ART

Parts, such as the friction plates of automatic transmissions ofautomobiles, are required to have an ability of suppressing thepropagation of cracks in under frictional heat conditions and to havehigh strength and hardness in addition to having crack resistance in ashearing process.

In the related art, as disclosed in Patent Document 1, a technique, inwhich low carbon steel or steel having various alloying elements isannealed by a recovery annealing method after a cold rolling process,has been applied to high-strength cold rolled steel sheets used forfriction plates or guaranteeing hardness. In addition, a method ofperforming a spheroidizing heat treatment on high carbon steel hasgenerally been used, and Patent Document 2 has proposed a technique inwhich cold rolling is performed twice, in a process of cold rolling,annealing, and cold rolling.

However, it is difficult to manufacture high-strength steel sheets usingthe recovery annealing method, and techniques using the spheroidizingheat treatment method and performing cold rolling twice incur highmanufacturing costs.

In addition, although alloying elements such as carbon (C), silicon(Si), manganese (Mn), molybdenum (Mo), or chromium (Cr), mainly used tomanufacture high-strength steel sheets through cold rolling, areeffective in improving the strength of steel sheets throughsolid-solution strengthening, the excessive addition of such elementscauses segregation of the elements and formation of non-uniformmicrostructures. In particular, ferrite transformation is markedlydelayed because of an increase in the hardenability of steel duringcooling, low-temperature phases (martensite and austenite) are formed,and grain boundaries become non-uniform. Thus, cracks increase during ashearing process, and if frictional heat is generated during operations,cracks may easily propagate and form defects.

In addition, if alloying elements such as titanium (Ti), niobium (Nb),or vanadium (V) used for additional strength improvements are improperlyadded, coarse carbides, nitrides, and precipitates are formed alonggrain boundaries, thereby increasing formation of cracks andfacilitating propagation of cracks during a shearing process.Furthermore, if frictional heat is generated on a sheared portion duringan operation, the propagation of cracks may more easily occur.

RELATED ART DOCUMENTS

(Patent Document 1) Application No: KR 1998-0059809

(Patent Document 2) Application No: DE 2005-031462

DISCLOSURE Technical Problem

Aspects of the present disclosure may provide a high-strength coldrolled steel sheet having high shear processability and capable ofsuppressing the formation of cracks in a shearing process and underfrictional heat conditions, and a method for manufacturing thehigh-strength cold rolled steel sheet.

Aspects of the present disclosure are not limited to the above-mentionedaspects. The above-mentioned aspects and other aspects of the presentdisclosure will be clearly understood by those skilled in the artthrough the following description.

Technical Solution

According to an aspect of the present disclosure, there may be provideda high-strength cold rolled steel sheet having high shearprocessability, the high-strength cold rolled steel sheet including, bywt %, carbon (C): 0.05% to 0.10%, silicon (Si): 0.01% to 0.5%, manganese(Mn): 1.2% to 2.0%, aluminum (Al): 0.01% to 0.1%, chromium (Cr): 0.005%to 0.3%, boron (B): 0.0003% to 0.0010%, molybdenum (Mo): 0.005% to 0.2%,phosphorus (P): 0.001% to 0.05%, sulfur (S): 0.001% to 0.01%, nitrogen(N): 0.001% to 0.01%, niobium (Nb): 0.005% to 0.08%, titanium (Ti):0.005% to 0.13%, vanadium (V): 0.005% to 0.2%, and a balance of iron(Fe) and inevitable impurities, wherein the high-strength cold rolledsteel sheet may satisfy Formulas 1 and 2 below, and may include at leastone of carbides, nitrides, and carbonitrides.

According to another aspect of the present disclosure, a method formanufacturing a high-strength cold rolled steel sheet having high shearprocessability may include: reheating a steel slab to a temperature of1200° C. to 1350° C., the steel slab including, by wt %, carbon (C):0.05% to 0.10%, silicon (Si): 0.01% to 0.5%, manganese (Mn): 1.2% to2.0%, aluminum (Al): 0.01% to 0.1%, chromium (Cr): 0.005% to 0.3%, boron(B): 0.0003% to 0.0010%, molybdenum (Mo): 0.005% to 0.2%, phosphorus(P): 0.001% to 0.05%, sulfur (S): 0.001% to 0.01%, nitrogen (N): 0.001%to 0.01%, niobium (Nb): 0.005% to 0.08%, titanium (Ti): 0.005% to 0.13%,vanadium (V): 0.005% to 0.2%, and a balance of iron (Fe) and inevitableimpurities, the steel slab satisfying Formulas 1 and 2 below; hotrolling the heated steel slab within a temperature range of 850° C. to1150° C. to form a hot rolled steel sheet; after the hot rolling,cooling the hot rolled steel sheet to a temperature of 550° C. to 750°C. and coiling the cooled hot rolled steel sheet; and after the coiling,pickling the hot rolled steel sheet and cold rolling the hot rolledsteel sheet at a reduction ratio of 60% to 70%.

2.0≤[Mn]+2.5[Mo]+1.5[Cr]+300[B]≤2.5  Formula 1:

0.2≤([Nb]/93+[Ti]/48+[V]/51)/([C]/12+[N]/14)≤0.5  Formula 2:

(In Formulas 1 and 2, each element symbol refers to the weight percent(wt %) of the corresponding element.)

The above-described aspects of the present disclosure do not include allaspects or features of the present disclosure. Other aspects orfeatures, and effects of the present disclosure will be clearlyunderstood from the following descriptions of exemplary embodiments.

Advantageous Effects

The present disclosure provides a high-strength cold rolled steel sheethaving high shear processability and capable of suppressing theformation of cracks in a shearing process and under frictional heatconditions in addition to having high strength and high hardness, and amethod of manufacturing the high-strength cold rolled steel sheet.

DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing values of Formulas 1 and 2 of examples.

BEST MODE

Embodiments of the present disclosure will now be described in detail.The disclosure may, however, be exemplified in many different forms andshould not be construed as being limited to the specific embodiments setforth herein. Rather, these embodiments are provided so that thisdisclosure will be thorough and complete, and will fully convey thescope of the present disclosure to those skilled in the art.

Hereinafter, a high-strength cold rolled steel sheet having high shearprocessability of the present disclosure will be described in detail. Inthe following description, the content of each alloying element is givenin wt %, unless otherwise specified.

The high-strength cold rolled steel sheet having high shearprocessability of the present disclosure includes, by wt %, carbon (C):0.05% to 0.10%, silicon (Si): 0.01% to 0.5%, manganese (Mn): 1.2% to2.0%, aluminum (Al): 0.01% to 0.1%, chromium (Cr): 0.005% to 0.3%, boron(B): 0.0003% to 0.0010%, molybdenum (Mo): 0.005% to 0.2%, phosphorus(P): 0.001% to 0.05%, sulfur (S): 0.001% to 0.01%, nitrogen (N): 0.001%to 0.01%, niobium (Nb): 0.005% to 0.08%, titanium (Ti): 0.005% to 0.13%,vanadium (V): 0.005% to 0.2%, and a balance of iron (Fe) and inevitableimpurities, wherein the high-strength cold rolled steel sheet satisfiesFormulas 1 and 2 below and includes at least one of carbides, nitrides,and carbonitrides.

Carbon (C): 0.05% to 0.10%

Carbon (C) is the most economical and effective element in strengtheningsteel, and as the content of carbon (C) increases, tensile strengthincreases by the effect of precipitation strengthening or an increase inthe fraction of bainite. However, if the content of carbon (C) is lessthan 0.05%, reactions with titanium (Ti), niobium (Nb), and vanadium (V)for forming precipitates are reduced, and thus, the effect ofprecipitation strengthening is low. Conversely, if the content of carbon(C) is greater than 0.10 wt %, coarse carbides are easily formed alonggrain boundaries, and shear processability is poor because fine cracksare formed along coarse carbide interfaces during a shearing process.Therefore, it may be preferable that the content of carbon (C) be withinthe range of 0.05 wt % to 0.10 wt %.

Silicon (Si): 0.01% to 0.5%

Silicon (Si) has a deoxidizing effect on molten steel and asolid-solution strengthening effect, and improves formability bydelaying the formation of coarse carbides. However, if the content ofsilicon (Si) is less than 0.01%, the effect of delaying the formation ofcarbides is low, and thus it may be difficult to improve formability.Conversely, if the content of silicon (Si) is greater than 0.5%, thesurface quality of the steel sheet may be markedly worsened because redscale may be formed on the surface of the steel sheet during a hotrolling process, and the ductility and weldability of the steel sheetmay also be worsened. Therefore, it may be preferable that the contentof silicon (Si) be within the range of 0.01% to 0.5%.

Manganese (Mn): 1.2% to 2.0%

Like silicon (Si), manganese (Mn) is an effective element having asolid-solution strengthening effect on steel and facilitating theformation of bainite in a weld heat affected zone after a weldingprocess by increasing the hardenability of the steel. However, if thecontent of manganese (Mn) is less than 1.2%, these effects may not besufficiently obtained by the addition of manganese (Mn). Conversely, ifthe content of manganese (Mn) is greater than 2.0%, hardenabilitymarkedly increases to result in a delay in ferrite transformation and adecrease in the effect of precipitation strengthening. In addition, whena slab is manufactured through a continuous casting process, segregationmarkedly occurs in a thickness wise center portion, and thus anon-uniform microstructure is formed in a thickness direction duringcooling after a hot rolling process, thereby markedly increasing cracksin a shearing process. Therefore, it may be preferable that the contentof manganese (Mn) be within the range of 1.2% to 2.0%.

Molybdenum (Mo): 0.005% to 0.2%

Molybdenum (Mo) has a solid-solution strengthening effect and increaseshardenability, thereby increasing the strength of steel. However, if thecontent of molybdenum (Mo) is less than 0.005%, these effects may not beobtained. If the content of molybdenum (Mo) is greater than 0.2%,hardenability excessively increases, thereby delaying ferritetransformation and decreasing precipitation strengthening. In addition,it has a negative effect on economical aspects and weldability.Therefore, it may be preferable that the content of molybdenum (Mo) bewithin the range of 0.01% to 0.2%.

Chromium (Cr): 0.005% to 0.3%

Chromium (Cr) has a solid-solution strengthening effect and increaseshardenability, thereby increasing the strength of steel. However, if thecontent of chromium (Cr) is less than 0.005%, these effects may not beobtained. Conversely, if the content of chromium (Cr) is greater than0.3%, ferrite transformation is excessively delayed, and thus martensiteis formed to result in poor elongation. In addition, the effect ofprecipitation strengthening is reduced. Furthermore, like in the case ofmanganese (Mn), segregation markedly increases in a thickness wisecenter portion, and a non-uniform microstructure is formed in athickness direction to result in poor sheer processability. Therefore,it may be preferable that the content of chromium (Cr) be within therange of 0.005% to 0.3%.

Boron (B): 0.0003% to 0.0010%

If even a small amount of boron (B) is added to steel, the hardenabilityof the steel improves. If the content of boron (B) is greater than0.0003%, boron (B) may segregate along austenite grain boundaries athigh temperatures, thereby stabilizing grain boundaries and improvingimpact resistance. However, if the content of boron (B) is less than0.0003%, these effects may not be sufficiently obtained. Conversely, ifthe content of boron (B) is greater than 0.0010%, elongated grainsincrease because recrystallization is delayed during a hot rollingprocess, and a non-uniform microstructure is formed because ferritetransformation is delayed during cooling. In addition, it may bedifficult to obtain an intended degree of strength as the effect ofprecipitation strengthening may decrease, and since a non-uniformmicrostructure of an initial hot rolled steel sheet may be a cause oflocal stress concentration in a cold rolling process, it is not favoredin the present disclosure. Therefore, it may be preferable that thecontent of boron (B) be within the range of 0.0003% to 0.0010%.

Phosphorus (P): 0.001% to 0.05%

Like silicon (Si), phosphorus (P) has a solid-solution strengtheningeffect and an effect of facilitating ferrite transformation. However, ifthe content of phosphorus (P) is less than 0.001%, it is noteconomically favorable, because of high manufacturing costs, andsufficient strength may also not be obtained. Conversely, if the contentof phosphorus (P) is greater than 0.05%, embrittlement may occur becauseof grain boundary segregation, fine cracks may easily be formed during ashearing process, and ductility and impact resistance may be worsened.Therefore, it may be preferable that the content of phosphorus (P) bewithin the range of 0.001% to 0.05%.

Sulfur (S): 0.001% to 0.01%

Sulfur (S) is an impurity existing in steel. If the content of sulfur(S) is greater than 0.01%, sulfur (S) may combine with an element suchas manganese (Mn) and form a non-metallic inclusion, therebyfacilitating the formation of fine cracks during a cutting process ofsteel and markedly decreasing stretch flangeability and impactresistance. Conversely, if the content of sulfur (S) is lower than0.001%, performing a steel making process may require an excessiveamount of time, and thus the productivity of the steel making processmay be lowered. Therefore, it may be preferable that the content ofsulfur (S) be within the range of 0.001% to 0.01%.

Aluminum (Al): 0.01% to 0.1%

In general, aluminum (Al) is added for deoxidation. If the content ofaluminum (Al) is less than 0.01%, this effect is insufficient.Conversely, if the content of aluminum (Al) is greater than 0.1%,aluminum (Al) may combine with nitrogen (N) and form AlN, therebyincreasing the possibility of cracks in slab corners during a continuouscasting process and the possibility of inclusion defects in edgeportions of a hot rolled steel sheet. In addition, surface defects maybe formed during a cold rolling process after a hot rolling process,thereby resulting in poor surface quality. Therefore, it may bepreferable that the content of aluminum (Al) be within the range of0.01% to 0.1%.

Nitrogen (N): 0.001% to 0.01%

Together with carbon (C), nitrogen (N) is a typical element having asolid-solution strengthening effect and forms coarse precipitatestogether with elements such as titanium (Ti) or aluminum (Al). Ingeneral, although nitrogen (N) has a solid-solution strengthening effectgreater than that of carbon (C), as the content of nitrogen (N)increases in steel, the toughness of the steel decreases. In addition,it takes an excessive amount of time to make steel having nitrogen (N)in an amount of less than 0.001%, and thus the productivity of a steelmaking process decreases. Therefore, according to the presentdisclosure, it may be preferable that the content of nitrogen (N) bewithin the range of 0.001% to 0.01%.

Titanium (Ti): 0.005% to 0.13%

Together with niobium (Nb) and vanadium (V), titanium (Ti) is a typicalelement having a precipitation strengthening effect and has a strongaffinity for nitrogen (N), forming coarse TiN in steel. TiN has aneffect of suppressing the growth of grains during a heating process forhot rolling. In addition, titanium (Ti) remaining after reaction withnitrogen (N) dissolves in steel and combines with carbon (C), therebyforming a TiC precipitate and thus improving the strength of the steel.However, if the content of titanium (Ti) is less than 0.005%, theseeffects may not be obtained, and if the content of titanium (Ti) isgreater than 0.13%, coarse TiN is formed to worsen shear processabilityduring a shearing process. Therefore, according to the presentdisclosure, it may be preferable that the content of titanium (Ti) bewithin the range of 0.005% to 0.13%.

Niobium (Nb): 0.005% to 0.08%

Together with titanium (Ti) and vanadium (V), niobium (Nb) is a typicalelement having a precipitation strengthening effect. During a hotrolling process, niobium (Nb) precipitates and delays recrystallization,thereby having a grain refinement effect and thus improving the strengthand impact toughness of steel. However, if the content of niobium (Nb)is less than 0.005%, these effects may not be sufficiently obtained.Conversely, if the content of niobium (Nb) is greater than 0.08%,recrystallization is excessively delayed during a hot rolling process,and thus shear processability deteriorates due to the formation ofelongated grains and coarse complex precipitates. Therefore, accordingto the present disclosure, it may be preferable that the content oftitanium (Ti) be within the range of 0.005% to 0.08%.

Vanadium (V): 0.005% to 0.2%

Together with niobium (Nb) and titanium (Ti), vanadium (V) is a typicalelement having a precipitation strengthening effect. Vanadium (V) formsprecipitates after a coiling process, thereby effectively improving thestrength of steel. However, if the content of vanadium (V) is less than0.005%, this effect may not be sufficiently obtained, and if the contentof vanadium (V) is greater than 0.2%, coarse complex precipitates areformed, thereby worsening shear processability and having aneconomically negative effect. Therefore, according to the presentdisclosure, it may be preferable that the content of vanadium (V) bewithin the range of 0.005% to 0.2%.

The other component of the high-strength cold rolled steel sheet of thepresent disclosure is iron (Fe). However, impurities of raw materials ormanufacturing environments may be inevitably included in thehigh-strength cold rolled steel sheet, and such impurities may not beremoved from the high-strength cold rolled steel sheet. Such impuritiesare well-known to those of ordinary skill in manufacturing industries,and thus specific descriptions of the impurities will not be given inthe present disclosure.

In the present disclosure, the composition of the high-strength coldrolled steel sheet may satisfy Formulas 1 and 2 below. Then, themicrostructure of the high-strength cold rolled steel sheet may beuniform after a hot rolling process, and the formation of cracks may besuppressed during a shearing process after a cold rolling process.

2.0≤[Mn]+2.5[Mo]+1.5[Cr]+300[B]≤2.5  Formula 1:

0.2≤([Nb]/93+[Ti]/48+[V]/51)/([C]/12+[N]/14)≤0.5  Formula 2:

(In Formulas 1 and 2, each element symbol refers to the weight percent(wt %) of the corresponding element.)

Formula 1 relates the hardenability and segregates of steel. Formula 1is proposed by considering a solid-solution strengthening effect onsteel and a non-uniform microstructure of steel.

If Formula 1 is less than 2.0, the solid-solution strengthening effecton steel is insufficient, and thus sufficiently high strength is notobtained. Conversely, if Formula 1 is greater than 2.5, themicrostructure of steel is non-uniform in a thickness direction of thesteel, and ferrite transformation is delayed, thereby decreasing theeffect of precipitation strengthening.

Therefore, it may be preferable that Formula 1 be within the range of2.0 to 2.5.

Formula 2 regulates elements relating to precipitates in steel. That is,since the formation of precipitates relates to the relationship betweenthe contents of Ti, Nb, and V, and the contents of C and N, Formula 2 isproposed to adjust the contents of Ti, Nb, and V according to thecontents of C and N.

If Formula 2 is less than 0.2, precipitation strengthening is markedlylowered, and thus, intended levels of strength and hardness may not beobtained. Conversely, if Formula 2 is greater than 0.5, fineprecipitates are formed in large amounts, thereby markedly increasingyield strength and thus worsening cold rollability. In addition,precipitates may be unevenly formed in the thickness direction of thesteel sheet, increasing the formation of cracks in a shearing processafter a cold rolling process.

Therefore, it may be preferable that Formula 2 be within the range of0.2 to 0.5.

If the high-strength cold rolled steel sheet is controlled to satisfythe above-described alloying composition, the high-strength cold rolledsteel sheet may have high shear processability and be capable ofsuppressing the formation of cracks in a shearing process and underfrictional heat conditions in addition to having high strength and highhardness.

The cold rolled steel sheet of the present disclosure is obtained byperforming a cold rolling process on a hot rolled steel sheet having amicrostructure of ferrite and fine pearlite, and thus the cold rolledsteel sheet has a microstructure (full hard microstructure) markedlydeformed in the rolling direction. In this case, each phase in themicrostructure of the cold rolled steel sheet may not be identified, butbefore the cold rolling process, the hot rolled steel sheet may includeferrite in an area fraction of 90% or greater, fine pearlite in an areafraction of less than 5%, and bainite as an inevitable remainder.

According to the present disclosure, the high-strength cold rolled steelsheet having high shear processability includes at least one ofcarbides, nitrides, and carbonitrides. For example, the high-strengthcold rolled steel sheet may include at least one of TiN, TiC, NbC, NbN,(Ti,Nb) (C,N), (Ti,Mo,Nb) (C,N), (Fe,Mn)₃C, and (Fe,Mn,Mo)C.

In this case, preferably, the average size of the carbides, nitrides,and carbonitrides may be within the range of 10 nm to 50 nm.

If the average size is less than 10 nm, the yield strength of the hotrolled steel sheet may be excessively high, thereby easily causing localwork hardening deviations during a cold rolling process, and easilycausing cracks during a shearing process and a heat treatment process ofthe cold rolled steel sheet.

Conversely, if the average size is greater than 50 nm, it is difficultto obtain intended tensile strength and hardness.

In addition, the cold rolled steel sheet may have a tensile strength of1200 MPa or greater and a hardness (micro-Vickers) of 340 Hv or greater.If the cold rolled steel sheet has tensile strength and hardness inthese ranges, the cold rolled steel sheet may be suitably used forapplications such as fiction plates of automatic transmissions ofautomobiles.

In addition, when a shearing process is performed on the cold rolledsteel sheet, the maximum length of cracks may preferably be 1 mm orless.

The maximum length of cracks is measured by punching the cold rolledsteel sheet using a circular die having a diameter of 10 mm with aclearance of 6%, heat treating the cold rolled steel sheet at 200° C.for 1 hour, and measuring the maximum length of cracks in across-section.

If the maximum length of cracks is greater than 1 mm, cracks may beformed in large amounts and may easily propagate during a shearingprocess, and the propagation of cracks may be facilitated if temperatureincreases due to frictional heat.

Hereinafter, a method for manufacturing a high-strength cold rolledsteel sheet having high shear processability will be described in detailaccording to another aspect of the present disclosure.

Another aspect of the present disclosure provides a method formanufacturing an high-strength cold rolled steel sheet having high shearprocessability, the method including: heating a steel slab having theabove-described alloying composition to a temperature of 1200° C. to1350° c.; hot rolling the heated steel slab within a temperature rangeof 850° C. to 1150° C. to form a hot rolled steel sheet; cooling the hotrolled steel sheet to a temperature of 550° C. to 750° C. and coilingthe hot rolled steel sheet; and pickling the coiled hot rolled steelsheet and cold rolling the pickled hot rolled steel sheet at a reductionratio of 60% to 70%.

Heating

A steel slab having the above-described alloying composition is heatedto a temperature of 1200° C. to 1350° C.

If the heating temperature is lower than 1200° C., sufficient amounts ofprecipitates may not redissolved. In this case, after a hot rollingprocess, the formation of precipitates decreases, and coarse TiNremains. Conversely, if the heating temperature is higher than 1350° C.,strength decreases because of abnormal growth of austenite grains. Thus,it may be preferable that the reheating temperature be adjusted to bewithin the range of 1200° C. to 1350° C.

In this case, the steel slab may be manufactured through a continuouscasting process directly connected to a hot rolling process.

Since the temperature of the steel slab is adjusted to be within therange of 1200° C. to 1350° C. so as to re-dissolve TiN, TiC, NbC, NbN,(Ti,Nb) (C,N), and (Ti,Mo,Nb) (C,N) precipitates, this manufacturingmethod may be used for the case in which a continuous casting process isdirectly connected to a hot rolling process as described above.

Hot Rolling

The heated steel slab is hot rolled within the temperature range of 850°C. to 1150° C.

If the hot rolling starts at a temperature higher than 1150° C., a hotrolled steel sheet having coarse grains and poor surface quality may bemanufactured because the hot rolling temperature is too high. Inaddition, if the hot rolling finishes at a temperature lower than 850°C., since recrystallization is excessively delayed, elongated grains aredeveloped, and yield strength increases, thereby worsening coldrollability and shear processability.

Cooling and Coiling

After the hot rolling, the hot rolled steel sheet is cooled to atemperature of 550° C. to 750° C. and is then coiled.

If the hot rolled steel sheet is cooled to a temperature less than 550°C. and is then coiled, the material quality of the steel sheet may bedegraded due to the formation of bainite and martensite. Conversely, ifthe hot rolled steel sheet is cooled to a temperature greater than 750°C. and is then coiled, coarse ferrite is formed, and coarse carbides andnitrides may easily be formed, thereby worsening the material quality ofthe steel sheet.

The average rate of the cooling may be within the range of 10° C./sec to70° C./sec.

If the average cooling rate is less than 10° C./sec, a non-uniformmicrostructure may be formed because of the formation of coarse ferrite.Conversely, if the average cooling rate is greater than 70° C./sec,bainite may easily be formed, and the microstructure of the steel sheetmay become non-uniform in the thickness direction, thereby worsening theshear processability of the steel sheet.

Cold Rolling

After the coiling, the hot rolled steel sheet is cold rolled at areduction ratio of 60% to 70% to manufacture a cold rolled steel sheet.

If the reduction ratio of the cold rolling process is less than 60%, theeffect of work hardening may not be sufficiently obtained, and thus itis difficult to guarantee the strength and hardness of the cold rolledsteel sheet. Conversely, if the reduction ratio of the cold rollingprocess is greater than 70%, the quality of the cold rolled steel sheetmay decrease at edge portions, and the shear processability of the coldrolled steel sheet may deteriorate.

The cold rolled steel sheet manufactured by the above-described methodmay have high strength and high hardness, and may not be easily crackedin a shearing process and under frictional heat conditions.

In addition, the cold rolled steel sheet manufactured by theabove-described method may include at least one of carbides, nitrides,and carbonitrides, and the average size of the carbides, nitrides, andcarbonitrides may be within the range of 10 nm to 50 nm. In addition,the cold rolled steel sheet may have a tensile strength of 1200 MPa orgreater and a hardness of 340 Hv or greater, and the maximum length ofcracks formed in the cold rolled steel sheet during a shearing processmay be 1 mm or less.

MODE FOR INVENTION

Hereinafter, the present disclosure will be described more specificallythrough examples. However, the following examples should be consideredin a descriptive sense only and not for purposes of limitation. Thescope of the present invention is defined by the appended claims, andmodifications and variations may be reasonably made therefrom.

Steel slabs having the compositions shown in Table 1 below were heatedto 1250° C., and cold rolled steel sheets were manufactured using theheated steel slabs under the conditions shown in Table 2 below. At thattime, the rate of cooling after a hot rolling process was adjusted to bewithin the range of 20° C./sec to 30° C./sec.

Furthermore, as shown in Table 2 below, values of Formulas 1 and 2 werecalculated for comparative examples and inventive examples, and FDT andCT refer to a finish rolling termination temperature in the hot rollingprocess and a coiling temperature, respectively.

In addition, Table 3 below shows mechanical properties and results ofmicrostructure observation of inventive examples and comparativeexamples. In Table 3 below, TS and Hv refer to the tensile strength andmicro-Vickers hardness of each cold rolled steel sheet, respectively,and a maximum length of cracks refers to a value measured by punchingeach cold rolled steel sheet using a circular die having a diameter of10 mm with a clearance of 6%, heat treating the cold rolled steel sheetat 200° C. for 1 hour, and measuring the length of the maximum crack ina cross-section. The length of cracks was measured from results ofobservation using an optical microscope at a magnification of 100 times.

The size of carbides, nitrides, and carbonitrides was analyzed using hotrolled steel sheets before cold rolling. Carbides, nitrides, andcarbonitrides having an average size of 10 nm to 50 nm do not vary insize and fraction after cold rolling, and it is difficult to accuratelymeasure the sizes and fractions of carbides, nitrides, and carbonitridesfrom microstructures markedly deformed after cold rolling. Thus, thesize thereof was analyzed using the hot rolled steel sheets. The averagesize of carbides, nitrides, and carbonitrides was determined fromresults of measurements obtained using a transmission electronmicroscope. Results of measurement at a magnification of 50,000 timeswere used to measure carbides and nitrides having an average size of 100nm or greater, and results of measurement at a magnification of 100,000times were used to measure precipitates having an average size of 100 nmor less. A tensile test was performed using specimens taken by JIS5 fromrolled steel sheets in a 0-degree direction with respect to the rollingdirection of the rolled steel sheets.

TABLE 1 Examples C Si Mn Cr Al P S N Mo Ti Nb V B *CE 1 0.04 0.2 1.60.01 0.03 0.01 0.004 0.004 0.005 0.1 0.04 0.005 0.0003 CE 2 0.05 0.1 1.40.1 0.03 0.008 0.003 0.004 0.007 0.06 0.03 0.1 0.0003 CE 3 0.06 0.1 1.90.2 0.035 0.01 0.003 0.005 0.05 0.08 0.03 0.005 0.001 CE 4 0.07 0.2 1.80.01 0.05 0.01 0.005 0.004 0.1 0.035 0.03 0.005 0.0003 CE 5 0.08 0.3 1.40.01 0.025 0.009 0.003 0.004 0.1 0.08 0.005 0.05 0.0003 CE 6 0.07 0.21.7 0.1 0.03 0.01 0.003 0.003 0.05 0.12 0.055 0.006 0.0003 CE 7 0.0850.1 1.8 0.01 0.05 0.01 0.004 0.004 0.009 0.09 0.04 0.005 0.0003 CE 80.09 0.05 1.9 0.1 0.025 0.007 0.003 0.004 0.007 0.11 0.035 0.05 0.0003CE 9 0.12 0.2 2 0.01 0.5 0.01 0.003 0.004 0.006 0.12 0.03 0.1 0.0003 CE10 0.08 0.5 2.2 0.01 0.1 0.01 0.003 0.004 0.05 0.15 0.03 0.02 0.0012**IE 1 0.055 0.1 1.7 0.01 0.03 0.01 0.003 0.005 0.1 0.06 0.02 0.0050.0003 IE 2 0.06 0.15 1.8 0.01 0.028 0.006 0.004 0.004 0.05 0.11 0.020.005 0.0003 IE 3 0.07 0.1 1.9 0.01 0.03 0.008 0.003 0.004 0.15 0.050.06 0.005 0.0003 IE 4 0.07 0.2 1.9 0.01 0.035 0.01 0.002 0.003 0.150.055 0.045 0.005 0.0003 IE 5 0.07 0.1 1.7 0.1 0.03 0.009 0.003 0.0040.05 0.08 0.03 0.05 0.0003 IE 6 0.07 0.4 1.9 0.01 0.037 0.01 0.003 0.0040.14 0.12 0.03 0.005 0.0003 IE 7 0.07 0.15 1.95 0.15 0.03 0.01 0.0040.003 0.05 0.1 0.02 0.005 0.0003 IE 8 0.075 0.2 1.8 0.01 0.03 0.0070.003 0.004 0.1 0.09 0.02 0.06 0.0003 *CE: Comparative Example, **IE:Inventive Example

TABLE 2 Reduction ratio of cold FDT CT rolling Examples Formula 1Formula 2 (° C.) (° C.) (%) *CE 1 1.72 0.72 887 605 66 CE 2 1.66 0.80889 614 73 CE 3 2.63 0.39 908 613 62 CE 4 2.16 0.19 892 615 65 CE 5 1.760.39 894 604 63 CE 6 2.07 0.53 895 612 62 CE 7 1.93 0.33 901 614 72 CE 82.16 0.47 898 608 56 CE 9 2.12 0.47 905 599 74 CE 10 2.70 0.55 911 61765 **IE 1 2.06 0.32 899 612 67 IE 2 2.03 0.49 905 610 66 IE 3 2.38 0.29909 623 62 IE 4 2.38 0.29 912 601 65 IE 5 2.07 0.49 915 625 65 IE 6 2.360.48 915 624 64 IE 7 2.39 0.40 918 626 68 IE 8 2.16 0.50 913 627 63 *CE:Comparative Example, **IE: Inventive Example

TABLE 3 Average size of Maximum length carbides, of cracks in Quality ofTarget nitrides, and sheared cross section TS TS Target carbonitridesportion of sheared Examples (MPa) Hv (MPa) Hv (nm) (mm) portion *CE 11237 334 ≥1200 ≥340 4 0.06 ⊚ CE 2 1260 341 7 4 Δ CE 3 1238 334 12 2 Δ CE4 1098 297 35 0.03 ⊚ CE 5 1236 334 13 0.08 ⊚ CE 6 1315 356 8 6 X CE 71372 362 ≥1350 ≥355 15 3 Δ CE 8 1291 349 13 0.05 ◯ CE 9 1434 384 22 8 XCE 10 1460 390 8 10 X **IE 1 1271 344 ≥1200 ≥340 15 0.06 ⊚ IE 2 1330 35316 0.2 ◯ IE 3 1263 342 18 0.4 ◯ IE 4 1281 346 19 0.05 ⊚ IE 5 1287 348 180.04 ⊚ IE 6 1384 368 ≥1350 ≥355 21 0.8 ◯ IE 7 1374 370 22 0.6 ◯ IE 81359 367 15 0.3 ◯ *CE: Comparative Example, **IE: Inventive Example

Comparative Examples 1 and 2 did not satisfy both Formulas 1 and 2, andComparative Example 1 had a carbon (C) content outside of the rangeproposed in the present disclosure. In both the comparative examples,sufficient solid-solution strengthening did not occur, and Formula 2exceeded the upper limit because of a relative low content of carbon (C)and relatively excessive contents of titanium (Ti), niobium (Nb), andvanadium (V). Therefore, although the size of carbides, nitrides, andprecipitates in steel was small, strength was insufficient. In addition,since Comparative Example 2 was prepared at a cold rolling reductionratio greater than the range proposed in the present disclosure,somewhat excessive cracks were formed in a sheared surface afterpunching, and the quality of the sheared surface was poor.

Comparative Examples 3 and 5 did not satisfy Formula 1. In ComparativeExample 3, the value of Formula 1 was greater than the range proposed inthe present disclosure, and thus, segregation increased in a centerportion of steel, resulting in poor quality in a sheared portion. Inaddition, Comparative Example 5 had very good quality on a shearedsurface because contents of elements such as manganese (Mn), chromium(Cr), and boron (B) were low, and thus, the occurrence of segregationwas low. However, solid-solution strengthening was not sufficient, andthus intended strength and hardness were not obtained.

Comparative Examples 4 and 6 did not satisfy Formula 2. In ComparativeExample 4, surplus carbon (C) remained and formed coarse precipitatesand carbides, and thus intended strength and hardness were not obtainedbecause of insufficient precipitation strengthening.

In Comparative Example 6, the value of Formula 2 was greater than therange proposed in the present disclosure, and thus fine precipitateswere formed in large amounts, thereby guaranteeing high strength butresulting in excessive cracking in a sheared portion.

Comparative Examples 7, 8, 9, and 10 were steel sheets manufactured tohave a tensile strength of 1350 MPa or greater and a hardness of 355 Hvor greater after a cold rolling process. Although Comparative Example 7did not satisfy Formula 1, Comparative Example 7 had intended physicalproperties owing to a high cold rolling reduction ratio. However,Comparative Example 7 had somewhat excessive cracks in a sheared portionbecause of the high cold rolling reduction ratio.

Comparative Examples 8 and 9 satisfied both Formulas 1 and 2, but thecold rolling reduction ratios thereof were improper, thereby failing toobtain intended physical properties or resulting in poor quality insheared portions. Comparative Example 10 did not satisfy both Formulas 1and 2 and had poor quality in a sheared portion.

However, inventive examples satisfying the composition, manufacturingconditions, and Formulas 1 and 2 proposed in the present disclosure hadintended material characteristics and good quality in sheared portions.

FIG. 1 illustrates the values of Formulas 1 and 2 and maximum cracklengths of sheared portions of inventive examples and comparativeexamples. In FIG. 1, a hatched region corresponds to the ranges proposedin the present disclosure.

While exemplary embodiments have been shown and described above, thescope of the present disclosure is not limited thereto, and it will beapparent to those skilled in the art that modifications and variationscould be made without departing from the scope of the present inventionas defined by the appended claims.

1. A high-strength cold rolled steel sheet having high shearprocessability, the high-strength cold rolled steel sheet comprising, bywt %, carbon (C): 0.05% to 0.10%, silicon (Si): 0.01% to 0.5%, manganese(Mn): 1.2% to 2.0%, aluminum (Al): 0.01% to 0.1%, chromium (Cr): 0.005%to 0.3%, boron (B): 0.0003% to 0.0010%, molybdenum (Mo): 0.005% to 0.2%,phosphorus (P): 0.001% to 0.05%, sulfur (S): 0.001% to 0.01%, nitrogen(N): 0.001% to 0.01%, niobium (Nb): 0.005% to 0.08%, titanium (Ti):0.005% to 0.13%, vanadium (V): 0.005% to 0.2%, and a balance of iron(Fe) and inevitable impurities, wherein the high-strength cold rolledsteel sheet satisfies Formulas 1 and 2 below, and the high-strength coldrolled steel sheet comprises at least one of carbides, nitrides, andcarbonitrides,2.0≤[Mn]+2.5[Mo]+1.5[Cr]+300[B]≤2.5  Formula 1:0.2≤([Nb]/93+[Ti]/48+[V]/51)/([C]/12+[N]/14)≤0.5  Formula 2: where eachelement symbol in Formulas 1 and 2 refers to a weight percent (wt %) ofa corresponding element.
 2. The high-strength cold rolled steel sheet ofclaim 1, wherein the carbides, the nitrides, and the carbonitrides havean average size within a range of 10 nm to 50 nm.
 3. The high-strengthcold rolled steel sheet of claim 1, wherein the high-strength coldrolled steel sheet has a tensile strength of 1200 MPa or greater and ahardness of 340 Hv or greater.
 4. The high-strength cold rolled steelsheet of claim 1, wherein a maximum length of cracks formed during ashearing process of the high-strength cold rolled steel sheet is 1 mm orless.
 5. The high-strength cold rolled steel sheet of claim 1, wherein amicrostructure of the high-strength cold rolled steel sheet before acold rolling process comprises ferrite in an area fraction of 90% orgreater, fine pearlite in an amount of less than 5%, and bainite as aninevitable remainder.
 6. A method for manufacturing a high-strength coldrolled steel sheet having high shear processability, the methodcomprising: reheating a steel slab to a temperature of 1200° C. to 1350°C., the steel slab comprising, by wt %, carbon (C): 0.05% to 0.10%,silicon (Si): 0.01% to 0.5%, manganese (Mn): 1.2% to 2.0%, aluminum(Al): 0.01% to 0.1%, chromium (Cr): 0.005% to 0.3%, boron (B): 0.0003%to 0.0010%, molybdenum (Mo): 0.005% to 0.2%, phosphorus (P): 0.001% to0.05%, sulfur (S): 0.001% to 0.01%, nitrogen (N): 0.001% to 0.01%,niobium (Nb): 0.005% to 0.08%, titanium (Ti): 0.005% to 0.13%, vanadium(V): 0.005% to 0.2%, and a balance of iron (Fe) and inevitableimpurities, the steel slab satisfying Formulas 1 and 2 below; hotrolling the heated steel slab within a temperature range of 850° C. to1150° C. to form a hot rolled steel sheet; after the hot rolling,cooling the hot rolled steel sheet to a temperature of 550° C. to 750°C. and coiling the cooled hot rolled steel sheet; and after the coiling,pickling the hot rolled steel sheet and cold rolling the hot rolledsteel sheet at a reduction ratio of 60% to 70%,2.0≤[Mn]+2.5[Mo]+1.5[Cr]+300[B]≤2.5  Formula 1:0.2≤([Nb]/93+[Ti]/48+[V]/51)/([C]/12+[N]/14)≤0.5  Formula 2: where eachelement symbol in Formulas 1 and 2 refers to a weight percent (wt %) ofa corresponding element.
 7. The method of claim 6, wherein the steelslab is manufactured through a continuous casting process.
 8. The methodof claim 6, wherein the cooling of the hot rolled steel sheet isperformed at an average cooling rate of 10° C./sec to 70° C./sec.
 9. Themethod of claim 6, wherein the cold rolled steel sheet comprises atleast one of carbides, nitrides, and carbonitrides, and the carbides,the nitrides, and the carbonitrides have an average size within a rangeof 10 nm to 50 nm.
 10. The method of claim 6, wherein the cold rolledsteel sheet has a tensile strength of 1200 MPa or greater and a hardnessof 340 Hv or greater.